The present invention relates to a hydrolytically stable amino-formaldehyde resin such as a urea and/or melamine-formaldehyde resin and to a process for preparing the resin.
Urea-formaldehyde condensation products are widely used in industry. They may be used as textile finishing agents, for example, to impart crease resistance. Ure-formaldehyde condensation products may also be utilized in fertilizers, as coatings, as insulation and in paper manufacture. One important use of these condensation products is as adhesives and binders, for example, in the manufacture of wood products such as particleboard and plywood.
Many particleboard plants are designed around the properties of urea formaldehyde (UF) resins. These resins have the virtues of low cost, rapid cure, processing convenience, and clear color. Very short press cycles can be achieved with urea formaldehyde adhesives; by adding a catalyst, the rate of cure can be adjusted to essentially any desired speed. Also, urea formaldehyde adhesives have "tack", causing adhesive-treated particles to stick to each other, so that mats made from a "tacky" furnish tend to be self-sustaining in shape, facilitating handling.
When particleboard was introduced in Europe, around 1950, resins with a relatively high formaldehyde content were used. For example, the resin synthesis described in J. A. Moore, Macromolecular Synthesis (Collective Volume 1), John Wiley & Sons (1977), uses a 1.85 formaldehyde to urea mole ratio. The final product has a viscosity of 150.varies.250 c.p., pH 7.5-7.7, and a free formaldehyde content between 2% and 4%. Despite this high formaldehyde content, the initial odor of the board did not cause any problems, because much of the material was used in furniture which was carefully finished and painted at the time. Currently, however, the pre-coated panels are cut and assembled with unfinished edges. Much formaldehyde emanates from these edges, where coarse wood facilitates its release. This slow release from the finished product imparts to the product an odor, which in many applications is considered objectionable. In addition, the UF resins contain methylol, methylene ether and other intermediate and reaction products which can readily and reversibly hydrolyse back to the initial raw material--formaldehyde. The weakest links are in the cellulose-resin link, the hemiacetals, ethers and methylols. The oxygen-free methylene linkage is the most resistant to hydrolysis. The degradation of the amine group increases with the number of protons on the nitrogen. Thus, tertiary cross-linked N-bonds are the strongest.
Formaldehyde release is especially noticeable in particleboard and in insulation foams, both of which contain cured resin films with a very large surface area which enhances formaldehyde release. The cause for formaldehyde release is complex. It can stem from a variety of partly related sources such as free, unreacted formaldehyde in the resin, and from formaldehyde dissolved in moisture on the wood product surface, where it readily dissolves. Its vapor pressure and its release rate change with changes in air humidity and product humidity. In particleboard, released formaldehyde can come from free formaldehyde which was bound to wood cellulose during the hot press cycle, and which slowly hydrolyzes under the influence of the acidic humidity in the wood. It can also result from the degradation of incompletely cured resin, or resin components, such as methylolurea. Finally it can result from bulk resin degradation.
The problem of formaldehyde release into the atmosphere is particularly aggravated where the release occurs inside a "tight" dwelling, so constructed to economize on energy, as many modern structures are.
Several paths have been explored over the last few years for reducing formaldehyde release. These include coating applications, chemical treatments before or after resin application, the use of resin additives, and new resin formulations. However, relatively little research has been conducted on new resin formulations.
The mole ratio of formaldehyde to urea has been slowly decreased over the years from its initial high value, but reduction in this ratio weakens the internal bond in particleboard, for example, even though it reduces the residual formaldehyde. A new generation of low odor resins is currently appearing, of which the syntheses are more carefully controlled. Some resin manufacturing operations are now programming formaldehyde (F) and urea (U) additions in two or more stages, to achieve a desired low F/U molar ratio. Other chemicals such as resorcinol and glyoxal have been used either to terminate the dimethylolurea or to react in the polymerization process, to reduce free formaldehyde.
The generally accepted procedure for making urea and formaldehyde resins is the reaction of urea and formaldehyde under alkaline conditions to form methylol ureas, followed by resinifying by further heating under acidic conditions and finally neutralizing and dehydrating to produce a product of the desired physical characteristics. This procedure requires very accurate control of the pH in the different stages of the process to prevent gelation, and it is at times difficult to obtain consistent physical properties.
Urea and formaldehyde will also react under various conditions of controlled acidity, but gelling systems are usually obtained. For example, if a mixture of one mole of urea and two moles of formaldehyde is maintained under acidic conditions, the mass gels unless steps are taken to interrupt the course of the reaction by the adjustment of the pH at the appropriate times.
In the common two stage, alkaline then acid reaction used for the commercial manufacture of urea-formaldehyde prepolymers, for use in adhesives, and in textile, paper and coatings, and in agricultural applications, or for solid resin moldings and other applications, the prepolymer resins are made by preparing a urea-formaldehyde solution having an F/U molar ratio ranging from 1.5 to 2.5. This solution is made basic with sodium hydroxide, triethanolamine, triethylamine, ammonia or any appropriate base that will establish a pH in the range of 7.5-8.9.
This basic solution is then brought to reflux for approximately 15-30 minutes, cooled slightly, and the pH is adjusted to a range of approximately 5.5-6.9 using formic acid, p-toluenesulfonic acid or other appropriate organic or inorganic acids. This acidic solution is then brought to reflux until a specific Gardner viscosity has been reached. At this predetermined viscosity point the temperature is dropped slightly, the resin adjusted to a pH of 7.2-7.6, and additional urea is added as required. Water is then removed under vacuum until a desired specific gravity is obtained or a desired percent solids reached. The resin is then cooled and ready for shipment as a prepolymer prior to final cure by the addition of acid.
This prepolymer commercial resin usually has a free formaldehyde content in the range of 0.5%-1.8%, but depending on the resin and its intended application, the free formaldehyde content may be as high as 5%. This common two stage manufacturing procedure results in a prepolymer resin containing methylol, dimethylene ether, and methylenediurea groupings.
The reactions which occur in such processes are of two kinds. The first involves the formation of methylol urea. Under the mildly alkaline conditions used in the first stage, both monomethylolureas and dimethylolureas are formed. The methylolurea reaction takes the form of: ##STR1## The second stage involves the condensation of methylolureas, under acidic conditions. In this stage, condensation occurs between methylol groups to form the dimethylene ether bridge: ##STR2## Reactions can also occur between the methylol group and the amido hydrogen of urea to form methylene bridges: ##STR3##
The polymerization or cure of such prepolymers normally goes through two distinct and separate stages. The first stage of cure involves formation of a low molecular weight fusible, soluble resin. The second stage of cure involves a reaction which converts the low molecular weight urea-formaldehyde resin into a high molecular weight network polymer. Cure is usually accomplished by heating under acidic conditions.
It has been postulated that various ether linkages in uncured resins further aggravate hydrolytic degradation in the cured state. There is also a large body of literature on the acid hydrolysis of compounds, having similar structures to that associated with the cured resin, which does demonstrate that the different linkages which may exist in cured UF resins could possess wide variations in hydrolytic stability. The following crude order of relative hydrolytic stabilities for possible links in a crude UF network has been postulated: methylene bridge&gt;dimethylene ether bridge&gt;methylol end group.
The formation of a hydrolytically stable urea-formaldehyde prepolymer is dependent on the manufacturing procedure involved in preparing the resin.
Many variations of these U-F condensation techniques have been tried, but there remains a need for a urea-formaldehyde resin which, when cured, will be characterized by low emission of formaldehyde, and will have equal, if not better, properties than those associated with current urea-formaldehyde resins.